1. Field of the Invention
The invention relates to a method for the nondestructive material testing of highly pure polycrystalline silicon.
2. Background Art
Highly pure polycrystalline silicon, referred to below as polysilicon, is used inter alia as a starting material for the production of electronic components and solar cells. It is produced on an industrial scale by thermal decomposition and CVD deposition of a gas containing silicon or a gas mixture containing silicon in Siemens reactors. The polysilicon is in this case formed as shaped polysilicon bodies. These can subsequently be processed mechanically.
These shaped polysilicon bodies must be tested by means of a nondestructive test method to assess their material quality. Acoustic resonance analysis, also known as a “sound test”, is generally used for this. The shaped polysilicon body is externally excited, for example by a gentle hammer blow, and the natural resonances resulting from this provide the person skilled in the art with information about the material quality of the shaped polysilicon body. One advantage of resonance is the very short test time of only a few seconds. The entire specimen body is furthermore studied during the test, i.e. the test is a volume-oriented test method. A disadvantage of resonance analysis is that accurate localization or material defect identification is not possible with this method. Moreover, the shaped polysilicon body is touched by a hammer blow and therefore contaminated in each test, which necessitates a subsequent cleaning step. Another disadvantage of resonance analysis is that the shaped polysilicon body may be damaged by the test. For example, superficial dislocations or even destruction of the shaped polysilicon body may take place. A further disadvantage is that the shaped polysilicon bodies differ in their shape, for example in diameter and length or in length, width and height or finished article geometry, and each shaped polysilicon body generates a different natural resonance owing to its different geometry. This makes comparison of the test results more difficult. Small defects which compromise the material quality, such as cracks, cavities or inclusions whose dimension is only a few millimeters large, cannot be detected by this test method.
Visual inspection is another nondestructive test method for polysilicon. The entire surface of the shaped polysilicon body to be tested is in this case assessed by a person skilled in the art. Visual inspection can be improved by various aids, such a special illumination systems or magnifying glasses. Although surface defects can also be identified and localized by visual inspection, here again material defects inside the shaped polysilicon bodies disadvantageously cannot be identified. The test is furthermore carried out by a person, i.e. the results are not objective and reliably reproducible; rather, they depend on the tester's “daily form” and experience.
For both said nondestructive test methods, furthermore, the shaped polysilicon body is handled with aids by the user. These aids, for example gloves, may become laden with dirt particles between two tests, which leads to contamination of the shaped polysilicon body during the test and necessitates a subsequent cleaning step.